package/clamav: add optional dependency to json-c

package/clamav: add optional dependency to json-c

clamav has optional support for json-c:

$ output/host/usr/bin/i586-buildroot-linux-uclibc-readelf -a output/target/usr/lib/libclamav.so.7.1.1 | grep NEEDED
 0x00000001 (NEEDED)                     Shared library: [libltdl.so.7]
 0x00000001 (NEEDED)                     Shared library: [libssl.so.1.0.0]
 0x00000001 (NEEDED)                     Shared library: [libcrypto.so.1.0.0]
 0x00000001 (NEEDED)                     Shared library: [libz.so.1]
 0x00000001 (NEEDED)                     Shared library: [libjson-c.so.2]
 0x00000001 (NEEDED)                     Shared library: [libc.so.0]

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>